Efforts for generating electricity at a high efficiency using coal have been made. As one of the efforts, there has been proposed a topping-cycle system which incorporates a pressurized fluidized-bed combustion furnace as shown in FIG. 14. In this system, coal is first gasified in a fluidized-bed gasification furnace 501. At this time, combustible components composed mainly of carbon (i.e., the so-called char produced in the gasification furnace 501) is combusted in a char combustion furnace 502 different from the gasification furnace 501. That is, a mixture of gas and char generated in the gasification furnace 501 is introduced into a cyclone 505 in which gas and char are separated from each other, and the char is supplied to the char combustion furnace 502 and the gas is supplied to a combustor 503. On the other hand, a fluidized medium and the char are supplied from the gasification furnace 501 to the char combustion furnace 502 in which the char is combusted to heat the fluidized medium, and the heated fluidized medium is returned to the gasification furnace 501. Combustible gas produced in the gasification furnace 501 and combustion exhaust gas generated in the char combustion furnace 502 are mixed with each other and combusted in the combustor 503 to raise the temperature, and then the exhaust gas is introduced into a gas turbine 504. Further, combustion exhaust gas and ashes generated in the char combustion furnace 502 are separated from each other in a cyclone 506, and as described above, the combustion exhaust gas is introduced into the combustor 503 and the ashes are discharged from the bottom of the cyclone.
Steam generated in the char combustion furnace 502 is introduced into a steam turbine 508, and then heated in a waste heat boiler 509, and then returned to the char combustion furnace 502. The combustion exhaust gas discharged from the gas turbine 504 is discharged from a stack 511 via the waste heat boiler 509.
As the temperature of gas at the inlet of the gas turbine is high, the efficiency of the gas turbine is high. Hence, in order to raise the efficiency of the total system, it is very important to keep the gas at the inlet of the gas turbine at a high temperature.
On the other hand, gasification performance is influenced by the kind of coal used. In general, as the reaction temperature is high, the reaction of gasification is accelerated, and the rate of gasification is increased. Therefore, in order to use various kinds of coal as fuel, it is extremely important to keep the temperature of the gasification furnace stable and high.
The methods for maintaining the temperature of the gasification furnace are roughly classified into two methods. One method is that a part of the fuel supplied to the gasification furnace is not gasified but combusted, and the other method is that char produced in the gasification furnace is supplied into the char combustion furnace together with the fluidized medium, for combusting thereby heating the fluidized medium, and the heated fluidized medium is returned to the gasification furnace. Generally, the combustion reaction rate of gas is much faster than the combustion reaction rate of solid as different order. Therefore, in the former case, most of the oxygen supplied to the gasification furnace reacts with gas component generated therein, and hence the yield of gas is lowered. In the latter case, since gas produced in the gasification furnace is not consumed for maintaining the temperature in the furnace, the yield of gas is high and available coal range is wide.
However, the latter method requires a technology for circulating a large amount of heating medium having a high temperature from the gasification furnace to the char combustion furnace. However, this requires a handling technology for particles having a high temperature and containing unburned substances, so that this encounters technically difficult problems. The reason why the topping-cycle system incorporating the pressurized fluidized-bed combustion furnace has not been put to practical use yet is that the technology for handling the particles having a high temperature and containing unburned substances has not been developed.
On the other hand, there has been proposed an attempt in which the char combustion furnace and the gasification furnace are disposed adjacent to each other to shorten conveyance distance of particles having a high temperature therebetween. In this technology, the char combustion furnace is provided adjacent to the gasification furnace, and immersed heat transfer tubes are disposed in the bed of the char combustion furnace.
As shown in FIG. 15, the heat transfer coefficient between the heat transfer tube in the fluidized-bed and the heating medium is almost constant irrespective of degree of fluidization of the fluidized medium, if the superficial velocity of fluidizing gas is two times larger than the velocity required for minimum fluidization of the fluidized medium. That is, the immersed heat transfer tubes in the fluidized-bed can collect a constant quantity of heat, regardless of the superficial velocity. Therefore, if the quantity of heat generated in the fluidized-bed is changed (for example, the amount of coal supplied to the furnace is changed according to load change), the temperature of the fluidized-bed is changed since the amount of heat transfer is constant.
In the topping-cycle system incorporating the pressurized fluidized-bed combustion furnace, it is important to keep the gas at the outlet of the gasification furnace and the gas at the outlet of the combustion furnace at desired high temperatures, respectively. In such a structure in which the char combustion furnace and the gasification furnace are disposed adjacent to each other, since the fluidized medium is circulated between the char combustion furnace and the gasification furnace, both of the furnaces have a relationship to each other, and hence the fluctuation of each bed temperature may cause fatal damage to a stable operation of the whole system.
As a method for suppressing the fluctuation of bed temperature in the char combustion furnace incorporating the immersed heat transfer tubes, the amount of oxygen in the fluidizing gas supplied to the char combustion furnace is changed in accordance with the load, whereby the amount of char to be combusted is changed for thereby controlling the bed temperature.
However, control of the amount of char to be combusted by controlling the amount of oxygen has a slow response speed, stable control is difficult, and the bed temperature runs out of control. As a result, there is a probability that the fluidized medium and ashes are melted and the fluidized-bed cannot be maintained, resulting in stoppage of the operation.
In order to keep the gasification furnace at a high temperature, it is necessary that the heating medium is heated by combusting char in the char combustion furnace and the heated heating medium having a high temperature is supplied to the gasification furnace, and hence the bed temperature of the char combustion furnace must be high. However, if the bed temperature of the char combustion furnace is excessively high, then clinker is formed. Therefore, it is necessary that the bed temperature is controlled within a designated range and the char combustion furnace has an excellent controllability of the bed temperature.
The easiest method for controlling the temperature of the char combustion chamber is supplying heating medium having a low temperature when the temperature of the char combustion chamber is increased. For example, the amount of fluidized medium required for lowering the bed temperature from 950° C. to 900° C. by 50° C., though depending on the temperature of the fluidized medium supplied, in a case where the temperature of the fluidized medium supplied thereto is 400° C., may be 50/(900-400)= 1/10 of the total amount of the fluidized medium. Conversely, if the bed temperature is decreased to a value lower than a set value, since the bed temperature is recovered by combusting char, there is nothing to do.
Therefore, if there is such a method for supplying the fluidized medium having a low temperature to the char combustion furnace, when necessary, while observing change of the bed temperature of the combustion furnace, then temperature control of the char combustion furnace can be easily achieved. In this case, it is important to discharge the fluidized medium from the char combustion furnace, equal to the amount of fluidized medium supplied thereto.
On the other hand, conventionally, in an atmospheric fluidized-bed boiler, coal is combusted in the fluidized-bed, and heat is recovered from the heated fluidized medium and combustion exhaust gas. FIG. 16 is a schematic view showing an example of a conventional atmospheric fluidized-bed boiler. The fluidized-bed boiler comprises a combustion furnace 601 and a heat recovery chamber 602 which are partitioned from each other by a partition wall 600. Heat transfer surfaces 603 for heat recovery from the fluidized medium are provided in the heat recovery chamber 602, and heat transfer surfaces 604 for heat recovery from combustion gas are provided in the freeboard. Steam produced by heat recovery through the heat transfer surfaces 603 and 604 drives a steam turbine 605.
Since coal characteristics are affected by the kind of coal, the combustion rate in the fluidized-bed is different, and the rate of the collected heat from the fluidized medium to the collected heat from combustion gas is different depending on the kind of coal.
Therefore, the proper arrangement of the heat transfer surfaces for heat recovery from the fluidized medium and the heat transfer surfaces for heat recovery from combustion gas is different depending on the kind of coal. Conventionally, the arrangement of the heat transfer surfaces in the boiler has heretofore been changed with every kind of coal. Therefore, there is a great limitation to change the kind of coal without reconstruction of the boiler, and if the kind of coal is greatly changed, the reconstruction of the boiler has been unavoidably conducted. This is because excessive heat transfer surfaces relative to the amount of heat recovery lead to lowering the furnace temperature, causing poor combustion and deteriorating the environment by combustion gas. Conversely, shortage of the heat transfer surfaces leads to raising the furnace temperature and causing clinker formation due to melting of ashes or agglomeration due to aggregation of the fluidized medium.